The present invention relates to a method for rejuvenating Ni-H.sub.2 batteries. More particularly, the invention relates to a method for rejuvenating Ni-H.sub.2 batteries used on satellites.
In recent years, Ni-H.sub.2 batteries have largely replaced Ni-Cd batteries for on-board satellite use due to numerous factors including improvements in specific energy, energy per unit volume, reliability and durability, as well as inherent overcharge and reversal protection. These batteries have found particular use in powering satellites that experience a loss of solar power for daily periods of a few minutes to more than an hour during the biannual eclipse seasons occurring at the time of the vernal and autumnal equinoxes.
An example of a single cell of a Ni-H.sub.2 battery is shown in cross section in FIG. 1. The cell is constructed within a cylindrical pressure vessel 1 having semispherical end portions. The pressure vessel 1 is formed by two halves joined by a weld ring 12. Inside the pressure vessel 1, an electrode stack 3 is sandwiched between a top end plate 10 and a bottom end plate 4. A positive bus bar 2 interconnects the positive electrodes of each element of the stack 3, while similarly a negative bus bar 11 interconnects the negative electrodes. A positive feedthrough conductor 9 is electrically connected with the positive bus bar 2 and extends through a seal 8 to the exterior of the pressure vessel 1, thereby forming a positive external terminal. Similarly, a negative feedthrough conductor 6 passes through a seal at the other end of the pressure vessel 1 and forms a negative external terminal. A fill port 7 is also formed with the negative feedthrough 6 to allow for initial filling of electrolyte and hydrogen gas into the pressure vessel 1. Reference numeral 5 in FIG. 1 indicates insulating washers.
As shown in FIG. 2, the electrode stack is composed of alternating positive nickel hydroxide electrodes 20 and negative hydrogen electrodes 21 separated by and in contact with an electrolyte matrix 22 saturated with a potassium hydroxide solution. The positive nickel hydroxide electrodes may be fabricated, for example, of 0.75 mm nickel sinters impregnated using a chemical, electrochemical or Fleischer method with Ni(OH).sub.2. The negative hydrogen electrodes may be formed of a film of Teflon-bonded platinum black 0.25 mm thick onto which is pressed a nickel screen. The back side of the electrode has a wet-proofing Teflon layer. The positive and negative electrodes are separated by a separator made, for instance, of asbestos. A gas diffusion mesh 23 made of a metal or plastic is disposed on the opposite side of the negative electrode.
The electrochemical reactions for the two electrodes are as follows: ##STR1##
The net reaction shows hydrogen reduction of nickelic hydroxide to nickelous hydroxide on discharge with no net change in KOH concentration or in the amount of water within the cell.
Further details concerning the construction and operation of these cells may be learned from, for example, U.S. Pat. No. 3,867,199 to Dunlop et al.; F. Betz et al., Nickel-Hydrogen Storage Battery for Use on Navigation Technology Satellite-2, 11th Intersociety Energy Conversion Engineering Conference, 1976; and J. D. Dunlop et al., Nickel-Hydrogen Battery Technology Development and Status, Comsat Technical Review, vol. 10, no. 2, fall 1980.
However, a heretofore unexplained anomaly has been observed in these batteries. Namely, there has been observed a degradation in voltage performance in some of the cells in multi-cell batteries.
For instance, a 27-cell Ni-H.sub.2 battery was submitted to a life test. Table 1 below lists the output voltages at the conclusion of nine eclipse seasons from five of the cells following a 72-minute discharge test together with the average voltages of the remaining 21 cells. The inferior performance of these five cells is apparent.
TABLE 1 ______________________________________ Eclipse Cell Cell Cell Cell Cell Normal Season No. 4 No. 17 No. 20 No. 25 No. 26 Cell Av. ______________________________________ 1 1.210 1.213 1.203 1.207 1.206 1.212 2 1.148 1.194 1.181 1.193 1.187 1.210 3 1.196 1.201 1.194 1.204 1.204 1.208 4 1.194 1.186 1.190 1.176 1.204 1.205 5 1.206 1.209 1.199 1.204 1.202 1.206 6 1.204 1.194 1.206 1.194 1.217 1.205 7 1.184 1.178 1.205 1.185 1.205 1.207 8 1.121 1.171 1.120 1.167 1.194 1.207 9 1.116 1.167 1.139 1.163 1.189 1.207 ______________________________________
Prior to the present invention, the reason for this degradation and a method for rejuvenating batteries containing cells suffering such a degradation were unknown.